1. Field of the Invention
The present invention relates to a flyback power converter with multiple outputs; in particular, to a multiple-outputs flyback power converter with a secondary side post regulator.
2. Description of Related Art
Low power converters with multiple outputs have been widely applied to a variety of electronic products such as monitors, mini PC, set-up box (STB), video and music player, game console, etc. Meanwhile, because of the soaring oil price and the increasing awareness of environmental protection, numerous energy-preserving measures have been rolling out.
For the purpose of output voltage regulation, the traditional method is to connect power resistors as dummy loads to each output terminal so that the output terminal may operate with the sufficient load and to have multiple output feedback arranged to stabilize the output signals. Another method is to incorporate linear voltage regulators to achieve the goal of regulating the output voltage at the output terminals other than the main output terminal. However, attending with the awareness of environmental protection, these methods cannot fulfill the demand of power efficiency.
FIG. 1 shows a flyback power converter with multiple outputs utilizing a linear regulator for stabilizing output voltage. Primary side of the flyback application circuit has a pulse width modulation (PWM) controller 11 and a power transistor 12. Secondary side of the flyback application circuit has a main output circuit and a secondary output circuit. Each output circuit has an output terminal VO1 or VO2 for outputting an output voltage. The PWM controller 11 on the primary side detects a voltage level of the main output terminal VO1 on the secondary side through a feedback device 13, and accordingly controls conduction time of the power transistor 12 so as to adjust electric energy stored in the primary side winding 140 of the transformer 14 from the power supplying terminal VIN. A voltage level of the main output terminal VO1 is thus controlled by the feedback circuit.
When the main output circuit utilizes the feedback device to regulate the voltage level of the output voltage, the second output circuit employs a linear regulator 22 to regulate a voltage level of the second output terminal VO2. However, limited by operations of the linear voltage regulator 22, a winding voltage provided by the second output winding 142 of the voltage converter 14 must be higher than the predetermined output voltage of the second output circuit to ensure the voltage level of the second output terminal VO2 could be in compliance with prevailing standards. The high winding voltage causes the increasing power consumption of the linear regulator 22.
Accordingly, how to offer stable multiple output voltages and reduce the power consumption at the same time is a problem to be addressed.